Those who rated themselves highly conscientious had an 89% lower risk of Alzheimer's than those who thought they were the least self-disciplined.

The study appears in the journal Archives of General Psychiatry.

This study only looked at one group of people and may not translate across to the whole population - not everyone can lead the lifestyle of a nun

Professor Clive BallardAlzheimer's Society

Critics warned against drawing conclusions for the general population, but the researchers said the group they studied, although all devoutly religious, contained a broad spectrum of personality types.

None of the participants had dementia when the study started in 1994, but 176 went on to develop the disease.

The researchers asked the volunteers to rank themselves on a five-point scale to determine just how conscientious they were. They also carried out medical and neurological tests each year until 2006.

The average score on the conscientious test was 34. Those who scored 40 or higher were found to be much less likely to develop Alzheimer's, and had a slower general rate of mental decline than those who scored 28 or lower.

When the researchers took into account a combination of risk factors, including smoking, inactivity and limited social connections, they still found that the dutiful people had a 54% lower risk of Alzheimer's compared to people with the lowest scores for conscientiousness.

Post mortem evidence

The researchers also analysed results from post mortem examinations of the brains of 324 participants who died during the study.

They found some evidence that conscientious people were less affected by the changes in the brain that are associated with Alzheimer's.

The researchers said it was unclear why conscientious people were less prone to Alzheimer's, but they suggested it may be that they tend to be more resilient, and better able to cope with life's difficulties.

They also postulated that such people tend to have a fair degree of success in their lives, and so were less affected by the chronic psychological stress associated with negative life events.

Professor Clive Ballard, director of research at the Alzheimer's Society, said the study did not prove any link between personality and brain changes associated with Alzheimer's.

He said: "We would exercise caution when translating these results.

"It is also important to remember that this study only looked at one group of people and may not translate across to the whole population. After all, not everyone can lead the lifestyle of a nun.

"However, we do know that there are many other things people can do to lower their risk of developing dementia. A healthy heart means a healthy brain so it's important to eat healthy, stay active and watch your waistline."

Rebecca Wood, of the Alzheimer's Research Trust, said it was difficult to prove conclusively which factors contributed to the disease.

However, she said: "Predicting those at risk will become increasingly important if future treatments and preventions are to be most effective."